1. Field of the Invention
The present invention relates to a lens injection- compression-molding method for injection-molding a thermoplastic resin to form a lens (mainly, a spectacle lens). More particularly, the present invention relates to a lens injection- compression-molding method capable of forming a lens at a high accuracy and a high quality.
2. Description of Related Art
An injection compression molding method is known as a method for forming a spectacle lens from a thermoplastic resin.
The injection compression molding method is a method of obtaining a spectacle lens by correcting the shrinkage of molten resin, clamping a die by leaving a margin for compression in a cavity for forming the spectacle, lens in order to obtain a uniform and high shape accuracy, then injecting molten resin into the cavity for forming the spectacle lens, and thereafter compressing the margin for compression.
To obtain a high-quality lens by the above injection compression molding method, it is important to control the position and speed of molten resin in the injection process. That is, it is important to accurately detect that the molten resin reaches the inside of a lens forming cavity and injecting the molten resin at a high speed and a constant speed. This is because, if the molten resin reaches the inside of the lens forming cavity and then the speed of the resin extremely changes in the cavity, the speed change in the cavity easily appears on the surface of a molded product as a wavy feed history.
Use of the closed control has been attempted so far to control the position and speed of molten resin in the injection process. That is, a method of detecting an actual injection speed to closed-control an injection speed while comparing the actual speed with a preset speed so that the deviation between the speeds is decreased to zero has been attempted.
In the case of lens formation, however, a time lag easily occurs between the movement of molten resin and a control system because an injection resistance is increased due to the shape of a lens or the flow property of molten resin.
Therefore, when using the closed control for all processes as ever, a problem occurs that an injection speed change easily appears on the surface of a molded product as a feed history because an injection speed is controlled so as to fluctuate little by little so that the deviation between an actual injection speed and a preset speed is decreased to zero.
The present invention is made to solve the conventional problem and its object is to provide a lens injection- compression-molding method making it possible to obtain a high-accuracy and high-quality lens.